The term avermectin (previously referred to as C-076) is used to described a series of compounds isolated from the fermentation broth of an avermectin-producing strain of Streptomyces avermitilis and derivatives thereof. The morphological characteristics of the culture are completely described in U.S. Pat. No. 4,310,519. The avermectin compounds are a series of macrolides, each of which is substituted at the 13 position with a 4-(.alpha.-L-oleandrosyl)-.alpha.-L-oleandrose group. The avermectin compounds and the derivatives of this invention have a very high degree of anthelmintic and anti-parasitic activity.
The avermectin series of compounds isolated from the fermentation broth have the following structure: ##STR1## wherein R.sub.4 is the 4'-(L-oleandrosyl)-.alpha.-L-oleandrosyloxygroup of the structure ##STR2## and wherein A at the 22,23 position indicates a single or a double bond; R.sub.1 is a hydrogen or hydroxy and is present only when A indicates a single bond;
R.sub.2 is iso-propyl or sec-butyl; and
R.sub.3 is methoxy or hydroxy.
There are eight different avermectin natural product compounds and they are given the designations A1a, A1b, A2a, A2b, B1a, B1b, B2a, and B2b based upon the structure of the individual compounds. In the foregoing structural formula, the individual avermectin compounds are as set forth below.
______________________________________ (The R group is 4'.alpha.(L-oleandrosyl).alpha.-L-oleandrosyloxy.) (A) R.sub.1 R.sub.2 R.sub.3 ______________________________________ A1a double bond -- sec-butyl --OCH.sub.3 A1b double bond -- iso-propyl --OCH.sub.3 A2a single bond --OH sec-butyl --OCH.sub.3 A2b single bond --OH iso-propyl --OCH.sub.3 B1a double bond -- sec-butyl --OH B1b double bond -- iso-propyl --OH B2a single bond --OH sec-butyl --OH B2b single bond --OH iso-propyl --OH ______________________________________
The avermectin compounds are generally isolated as mixtures of a and b components. Such compounds differ only in the nature of the R.sub.2 substituent and the minor structural differences have been found to have very little effect on the isolation procedures, chemical reactivity and biological activity of such compounds.
In addition to these natural avermectins containing the 25-iso-propyl or 25-sec-butyl-substituent, closely related derivatives containing other branched or cyclic 25-alkyl or 25-alkenyl substituents, including those further substituted by heteroatoms such as oxygen, sulfur, nitrogen, and halogen, are known in the literature. These derivatives are obtained through various adjustments and additions to the fermentation procedures as described fully in the European Patent Application EPO 0 214 731.
Avermectins are products of microbial fermentations using the actinomycete Streptomyces avermitilis. These microbes use acetates and propionates as building blocks for most of the avermectin carbon chain, which is then further modified by microbial enzymes to give the completed avermectin molecules. It is known, however, that the carbon C-25 and the 2-propyl and 2-butyl substituents at this carbon are not derived from acetate or propionate units, but are derived from the amino acids L-valine and L-isoleucine, respectively. It was reasoned that these amino acids are deaminated to the corresponding 2-ketoacids, and that these then are decarboxylated to give 2-methylpropionic and 2-methylbutyric acids. These acids then have been found to be directly incorporated into the avermectin structures to give the 2-propyl and 2-butyl C-25 substituents, as is reported by Chen et al., Abstr. Pap. Am. Chem. Soc. (186 Meet., MBTD 28, 1983). It was also disclosed in European Patent Application number 0 214 731 that additions of large amounts of other acids such as cyclopentanoic, cyclobutyric, 2-methylpentanoic, 2-methylhexanoic, thiophene-3-carboxylic acids and others to the fermentation broth of S. avermitilis causes the microbes to accept these acids as substitutes and to make small amounts of avermectins containing these acids in form of new C-25 substituents. Examples of such new avermectin derivatives are:
25-(thien-3-yl)-25-de-(1-methylpropyl)avermectin A2a PA0 25-(cyclohex-3-enyl)-25-de-(1-methylpropyl)avermectin A2a PA0 25-cyclohexy-25-de-(1-methylpropyl)avermectin A2a PA0 25-(1-methylthioethyl)-25-de-(1methylpropyl)avermectin A2a PA0 25-(2-methylcyclopropyl)-25-de-(1-methylpropyl)avermectin A2a.
Similar experiments producing avermectins "c" and "d" containing as C-25 substituents a 2-pentyl and 2-hexyl group are described by T. S. Chen et al. in Arch. Biochem. Biophys. 1989, 269, 544-547.
Still additional avermectin derivatives are produced through artificial modification of the fermentation of Streptomyces avermitilis either by addition of metabolic inhibitors such as sinefungin (as described by Schulman et al., J. Antibiot. 1985, 38, 1494-1498) or by mutation of the parent strain (as described by Schulman et al., Antimicrobial Agents and Chemotherapy, 1987, 31, 744-747, and by EP-276-131-A to Pfizer INC.). Some of these avermectin derivatives are still further modified and are missing one or two of the 3'- and 3"-O-methyl groups (Schulman et al., J. Antibiot. 1985, 38, 1494-1498).
The fermentation products have been chemically modified in order to obtain further antiparasitic and insecticidal analogs with improved properties. Publications of such procedures in the scientific and patent literature have been reviewed by Fisher, M. H.; Mrozik, H.; in Macrolide Antibiotics; Omura, S., Ed.; Academic: New York, 1984; pp. 553-606, and by Davies, H. G.; Green, R. H. Nat. Prod. Rep., 1986, 3, 87-121.
For example, a group of semisynthetic avermectin derivatives were obtained by hydrogenating specifically the 22,23-double bond of avermectin B1 giving 22,23-dihydroavermectin B1 derivatives which have very potent anthelmintic and antiparasitic properties. Other examples of semisynthetic avermectin derivatives contain a 8,9-oxide group, a 4.alpha.-hydroxy or acyloxy group, a 23-keto group, which all are potent antiparasitic and insecticidal compounds.
It has also been described by Mrozik in U.S. Pat. No. 4,427,663 that amino substituents at the 4"- and 4'-positions have very high antiparasitic and insecticidal activities.
These compounds may be used as starting materials for the compounds of this invention without further modification, or when containing additional reactive groups, which are not to be modified under the reaction conditions applied, only after protection of such with a suitable protecting group.